Prior art optical tops are traditionally made with internal mass-spring dampers to control structural resonances. These mass-springs are tuned, when the optical tops are assembled, to the primary bending freguencies of the optical tops. Energy is dissipated when the mass-spring assemblies are submerged in a viscous fluid.
This structure works well for optical tops that are lightly loaded. When a heavy load is applied to such optical tops, the apparent primary bending frequency is reduced and the built-in dampers are no longer tuned for maximum effect.
When two or more standard optical tops are bolted together, the standard individual unit dampers do not work well.
Traditional bolted or welded together optical tops are made without internal dampers until the tops have been bolted together. Then the assembled structure is tested and new, lower frequency dampers will be assembled in the finished table assembly. Again, to work at peak performance levels, those joined table structures must be used in the assembled state. If these assembled tops are taken apart, then the unit dampers do not work well.
Broadly the invention comprises an optical top or vibration isolated structure which has at least one accessible variable damper secured to the optical top. The damper is located such that it may be tuned without either disassembly of the top at the site or it can be returned to the manufacturer for disassembly and retuning.
The invention utilizes an accessible adjustable mass damper that can be tuned to one or more frequencies by the user as requirements change. Accessible is used in the sense that the damper can be adjusted without disassembling the optical table, i.e. removing the top and/or bottom-plates and/or the sides.
If a particularly heavy load is applied to an optical top reducing the resonant frequency, the adjustable damper's natural frequency is reduced by tuning the variable damper and peak performance is maintained. If two perfectly tuned tables are bolted together to form a unitary assembly, then again, the variable damper (or dampers) is adjusted to coincide with the new, lower structural frequencies of the assembly.
In a preferred embodiment of the invention, the optical table top has four dampers. The dampers in the optical table can be set up such that all four dampers are adjustable or some may be fixed while the rest are adjustable. At least one damper must be adjustable but, there is no limit to the number of dampers used. Where four dampers are used, they are preferably located at the corners of the optical top. In the preferred embodiment, threaded rods are used to adjust and lock a suspended mass which is submerged in a viscous liquid.